FROMLIST: KVM: arm64: Hook up ->page_count() for hypervisor stage-1 page-table

kvm_pgtable_hyp_unmap() relies on the ->page_count() function callback
being provided by the memory-management operations for the page-table.

Wire up this callback for the hypervisor stage-1 page-table.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20211201170411.1561936-6-qperret@google.com
Bug: 209599700
Change-Id: Ieaf1f60698e1ebafc60424e879ccfd6ec192dbb5